linux-kernel - Re: [PATCH] iio: kx022a: Fix raw read format

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Message-ID: <20241030180658.00000ac2@Huawei.com>
Date: Wed, 30 Oct 2024 18:06:58 +0000
From: Jonathan Cameron <Jonathan.Cameron@...wei.com>
To: Matti Vaittinen <mazziesaccount@...il.com>
CC: Matti Vaittinen <matti.vaittinen@...rohmeurope.com>, Kalle Niemi
	<kaleposti@...il.com>, Jonathan Cameron <jic23@...nel.org>, "Lars-Peter
 Clausen" <lars@...afoo.de>, Andy Shevchenko
	<andriy.shevchenko@...ux.intel.com>, <linux-iio@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iio: kx022a: Fix raw read format

On Wed, 30 Oct 2024 15:16:11 +0200
Matti Vaittinen <mazziesaccount@...il.com> wrote:

> The KX022A provides the accelerometer data in two subsequent registers.
> The registers are laid out so that the value obtained via bulk-read of
> these registers can be interpreted as signed 16-bit little endian value.
> The read value is converted to cpu_endianes and stored into 32bit integer.
> The le16_to_cpu() casts value to unsigned 16-bit value, and when this is
> assigned to 32-bit integer the resulting value will always be positive.
> 
> This has not been a problem to users (at least not all users) of the sysfs
> interface, who know the data format based on the scan info and who have
> converted the read value back to 16-bit signed value.
They shouldn't be doing that. Scaninfo is for buffered values only
This should indeed be signed.

> 
> This, however, will be a problem for those who use the in-kernel
> interfaces, especially the iio_read_channel_processed_scale().
> 
> The iio_read_channel_processed_scale() performs multiplications to the
> returned (always positive) raw value, which will cause strange results
> when the data from the sensor has been negative.
> 
> Fix the read_raw format by casting the result of the le_to_cpu() to
> signed 16-bit value before assigning it to the integer. This will make
> the negative readings to be correctly reported as negative.
> 
> This fix will be visible to users by changing values returned via sysfs
> to appear in correct (negative) format.
> 
> Reported-by: Kalle Niemi <kaleposti@...il.com>
> Fixes: 7c1d1677b322 ("iio: accel: Support Kionix/ROHM KX022A accelerometer")
> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
> Tested-by: Kalle Niemi <kaleposti@...il.com>
> ---
>  drivers/iio/accel/kionix-kx022a.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/accel/kionix-kx022a.c b/drivers/iio/accel/kionix-kx022a.c
> index 53d59a04ae15..b6a828a6df93 100644
> --- a/drivers/iio/accel/kionix-kx022a.c
> +++ b/drivers/iio/accel/kionix-kx022a.c
> @@ -594,7 +594,7 @@ static int kx022a_get_axis(struct kx022a_data *data,
>  	if (ret)
>  		return ret;
>  
> -	*val = le16_to_cpu(data->buffer[0]);
> +	*val = (s16)le16_to_cpu(data->buffer[0]);
LGTM.
>  
>  	return IIO_VAL_INT;
>  }
> 
> base-commit: 81983758430957d9a5cb3333fe324fd70cf63e7e